The present invention relates to apparatus for transmitting an encrypted information signal including an analog component and receiving apparatus for receiving such an encrypted signal and unscrambling it.
The use of earth satellites to radiate information services, including broadcast signals, may require the use of secure coding techniques which in this specification will be called encryption. An example of such a system would be subscription broadcast services, but the techniques to be described later could be suited equally well to any other information distrubtion system where controlled access is required. The issue of licences or other purely administrative means of controlling receivers does not fully guarantee that licencees are the only possible recipients of the signal or information. It is therefore particularly important that a guarantee of privacy be supported by some technical means of restricting access to an exclusive broadcast program or other information source.
The essence of a scrambling or encryption system is that, for the authorized recipient, the guarantee offered is of high integrity, that the hardware involved is simple and inexpensive and program or information reception by unauthorised recipients is both difficult and time consuming.
A number of different techniques exist whose complexity, convenience and cost of implementation vary. These techniques also vary in the degree of security which they afford; for example whether they render broadcast program material totally unintelligible or whether they simply impair the program sufficiently to render it worthless.
For the particular case of broadcast signals, audio, video and text signals can be present singly or in combination. Novel modulation schemes such as the multiplex analog components (MAC) system which is disclosed in our co-pending British application No. 8202622 is one such broadcast signal which has in any one line period three distinct blocks of information namely a digital block and first and second analog blocks containing video information. While it is possible to provide an effective encryption algorithm for the digital block using digital methods it is much more difficult to provide an effective encryption method for the analog portions of the signal because re-ordering of the analog video information in a line is only permissible if done within limits since any wholesale re-ordering affects the signal and noise spectra adversely at the receiver. It is an object of the present invention to provide apparatus for encrypting an information signal including an analog component in such a way that there is effectively no impairment of the received signal due to the encryption.